This invention relates to a method of controlling a duty ratio with which an on-off type solenoid valve is to be opened, and more particularly to a method of this kind which is adapted to perform control of an on-off type solenoid valve in a manner parallel with control of one or more other controlling means, with the latter having priority to the former control, by means of a microcomputer.
In controlling the duty ratio of opening of an on-off type solenoid valve, i.e., the pulse duty factor of a driving signal for driving the valve, for example, in controlling the duty ratio of opening of a valve of this kind for regulating an exhaust gas recirculation amount in an internal combustion engine (hereinafter called "the EGR control valve"), in a manner parallel with control of another controlling means, for instance, a fuel injection device, with the latter having priority to the former, by means of a single microcomputer, i.e. an electronic control unit (hereinafter called "the ECU"), if an interrupt signal commanding execution of the control of the fuel injection device and an interrupt signal commanding execution of the control of the EGR control valve are inputted to the ECU almost at the same time, the ECU executes the control of the fuel injection device prior to the control of the EGR control valve, and after completion of the execution of the former control, it executes the control of the latter. This manner of priority control can often result in irregularities in the pulse repetition period of the driving signal for the EGR control valve. As a consequence, when the pulse duty factor of the driving signal is set to a value close to 100%, it can happen that adjacent pulses of the driving signal are generated in a manner substantially continuous with each other. This causes the EGR control valve to open with a substantial duty ratio larger than the actually required duty ratio, resulting in an inaccurate exhaust gas recirculation amount.